


Red Oni, Blue Oni

by Tangerine_Catnip



Category: Undertale (Video Game)
Genre: Brother/Brother Incest, Depression, Dom Papyrus, Guilt, M/M, Romantic Comedy, Soul Bond, Soulmates, Sub Sans, Unresolved Sexual Tension
Language: English
Status: Completed
Published: 2015-11-26
Updated: 2015-11-26
Packaged: 2018-05-03 10:19:13
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,098
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5286947
Author URL: https://archiveofourown.org/users/Tangerine_Catnip/pseuds/Tangerine_Catnip
Summary: <blockquote class="userstuff">
              <p>Some pairs go together so well you could hardly imagine one without the other. Likewise, sans never contemplated living without Papyrus. But situations change, and now sans has to decide between braking their perfect partnership, or risking doing something he'll regret for the rest of his life.</p>
            </blockquote>





	Red Oni, Blue Oni

**Author's Note:**

> This fic had a lot more structure before Papyrus started being Papyrus all over it.

Sans did a lot of things for his brother. Small things, big things. Many that Papyrus didn't even realize were him. (Though, sans secretly suspected that, sometimes at least, Papyrus did know it was him and was playing along. His Santa costume really wasn't that good.)

But no matter how many gifts he snuck under trees, or elaborate games he set up to keep his brother entertained, Sans could never even come close to repaying the debt he owed to Papyrus.

Usually, when one person owes their life to another, there's a story about how person A pushed person B out of the way of a train. Or maybe jumped in front of a bullet. But Papyrus had done it subtlety, so subtlety, he didn't even know about it.

All monsters, including sans, were made primarily of magic and positive emotions. So an absence of positive emotions was, to put it lightly, extremely frickin dangerous. Just as dangerous as going without food or water would be for a human. If a monster's soul lacked the will to sustain itself, it would wither and crumble into dust.

Death by Heartache took on a literal meaning in the underground. It was one of the reasons the exile from the surface was such a cruel punishment. Being trapped sucked the hope out of many monsters.

It was better now, since most either couldn't remember the surface or weren't alive when the barrier went up. But for many decades following the exile, the longing for the sun, the stars, and wide open spaces drove many to despair.

Sans knew exactly what that was like. The soul crushing despair. It was the most horrible feeling he had ever experienced. In that, it was the complete absence of any emotion at all.

It drained all your energy, then twisted your own thoughts against you. It ate at you from the inside out, then convinced you it was better this way.

Fortunately for monster-kind there was a built in defense mechanism. The emotions of others monsters could bleed over, flowing from one monster soul to the next. As long as the majority of the population had hope, there was always something to live for.

Humans had exploited this as a weapon of war, but in the safety of the underground, it was one of the few things that held the population together.

But even then, the bleed over effect wasn't usually enough to save a monster who had fallen into a depression as deep as sans had. There was a point where it was simply kinder to die and not drag others down with you.

Sans had waited for almost two years now, keeping a close eye on Papyrus. Waiting to see if he showed signs of suffering from the darkness that plagued his brother. But Papyrus' enthusiasm for life and everything else, never ever wavered.

Meanwhile the effect being near Papyrus had on sans was dramatic. Not dramatic enough to have him jumping out of bed every morning and doing a hundred push ups. But enough to keep him alive, and even more importantly, enough to keep him living.

It was because of Papyrus that sans got out of bed every morning, it was because of Papyrus he had the strength to keep playing his part over and over. Even though he knew nothing he ever did, or ever would do, really mattered.

Papyrus' positivity filled him up, even when they were just standing side by side for a moment. Like tossing fresh wood on the smouldering embers of a fire.

sans wasn't even sure if Papyrus fully understood what he was doing. He wondered aloud once or twice what sans would do without a cool brother to watch out for him. But it was hard to tell if that was his ego, or if he actually had any idea how much sans depended on him.

Lately, though, this perfect harmony had hit a tiny little snag. Well, a huge snag. An elephant sized snag sitting on sans' ribcage and making it impossible to breath.

See, as it turned out, the line between dependency and addiction was a little thinner than sans had previously assumed.

It started as a small craving, but got worse with every passing month. Sans would make up excuses to follow his brother on patrols, even though they were supposed to be taking turns watching the sentry point.

Then, when they were sitting on the couch, sans got into the habit of inching closer till Papyrus was squished at one end. Then, he started following Papyrus to cooking lessons with Undyne and waited outside until they were finished. Even though it required sitting through all the screaming, and potentially being used as a taste tester.

It took a while for sans to admit to himself he had a problem. But when the penny dropped. The penny dropped hard. Ringing to the tune of a long, breathy, moan pulled from Papyrus in the middle of a completely platonic brotherly tickle war.

sans couldn't exactly explain how his hands ended up on Papyrus' hips. Or why he had decided to run his fingers over the smooth ridges in a manner that was decidedly not tickling.

The mistake in and of itself was bad enough. But what shocked sans into his terrible revelation, was his reaction to the mistake. He loved it. Absolutely loved it. So much so, even sans' years of experience ignoring his problems couldn't make that memory go away.

sans was used to cravings. He had many that he acted on frequently. Usually for greasy food or naps. But the desire he felt in that moment made all of those feel like idle whims.

sans still didn't know how he managed to pull away, but as soon as he did, he hauled his bony backside as far away from Papyrus as possible, as fast as possible. Though, not fast enough to miss the look of disappointment on Papyruses face. It was just like him to only notice the fact he was missing out on a fun game to play, and overlooking tiny details, like whether or not his brother was putting his hand down his pants.

No matter how sans sliced it, he was damned if he did and damned if he didn't. Ignoring how his own mental state disintegrated when separated from his brother. Papyrus was also going stir crazy without his best friend around.

It had only been a week since sans had started avoiding Papyrus and it was already taking a devastating toll. Papyrus had been late for his sentry shift not just once, but twice. Gossip sprung up all over town as people speculated about what could have upset him so much.

It hurt. Even more than the clawing emptiness. Sans could handle his own pain, but the collateral damage was impossible to ignore.

Sans sat with his back to his bedroom door. He had been stewing in his own thoughts for a while and hadn't come up with any solutions. If he kept going as if nothing had happened, it would feed his addiction, and guarantee another lapse in judgement. He could force moderation, but then he would be half desperate, and the time he did spend with Papyrus would be twice as risky.

This would be so much easier if he had done what was best for everyone, and found a quiet spot to lay down and die. Instead of letting his bond with Papyrus get so strong neither of them could live without the other.

Sans shook his head, trying to throw off that train of thought. The voice telling him that wasn't his. The darkness was trying to pull him down. He could still fight it, but it wouldn't be long before he forgot and started to feel as if everything that little voice said was true.

This first week had been hard, but Papyrus would probably recover. Maybe the problem was he was doing it too slowly. If he stretched out the process it would just be more painful in the long term.

sans could disappear if he needed to, he had made preparations for it. Though, he never planned to use them to hide from Papyrus.

Out of all the options, he was leaning towards that one. But Sans knew better than to make that decision without giving it proper time to stew. He needed to feel like himself again, so he knew he was the one deciding.

sans hauled himself up into a standing position. He wavered for a moment, eyeing the ground as he felt a serious urge to flop down onto it and forget about the whole 'moving around' thing.

The thought of spending a moment in Papyrus' presence soon overpowered that notion. It wasn't a guarantee, sans had only decided to look and see if this might be a good moment. If not, he was going right back onto the floor. No matter how much he tried to convince himself otherwise.

sans pushed his door open and took the three steps up to the railing overlooking the living room. He moved so silently he could have snuck up on a ghost. The lights were all off, and the windows were dark. Sans must have been brooding for longer then he thought if it was night time.

Usually, Papyrus went to bed as soon as the sun set, and rose with it in the morning. Today, though, it seemed he hadn't quite made it to his room.

Papyrus lay stretched out on the lime green sofa. One orange boot was resting on the backrest while the other hung off the edge and onto the floor. He had an arm resting over his eye sockets, even though the room was dark.

Sans felt a rush of excitement. A sleeping Papyrus was easily the best outcome he could have hoped for. He wouldn't have snuck into his brother's room, but if he was taking a nap out in the open, there couldn't be any harm in just sitting next to him.

sans took the steps two at a time, then realizing what he was doing, he slowed down. He didn't want to wake Papyrus. If he did, there would no doubt be questions, not to mention heartbroken gazes.

His slippers muffling most of the sound he made, sans tiptoed over to the sofa and sat down next to the armrest Papyrus was using as a pillow.

It only took a few seconds for the heat to sink in. sans took a deep breath, the reflexive motion spreading the feeling throughout his body. The fog fell away, and the whole world came into sharp relief. A static filled background noise dissolving into a pleasant melody.

sans hadn't noticed the patterns of frost on the windows before. It was gorgeous. Tiny crystalline branches stretching out like vines over the glass. The stillness of the night settled like a warm blanket and sans could appreciate every moment of this blissful peace. Instead of watching the minutes drag on like hours and the days march in lock step one after the other.

No wonder he was getting addicted to this. Papyrus' world was filled with so much light and happiness. Pure night and day compared to him.

sans couldn't even managed to work up enough guilt to make himself feel bad for leaching off his brother when he was unconscious. He tucked his knees up under his chin and pressed his forehead against his kneecaps.

"Nhya ahah ha! I got you now, brother!"

sans leapt to his feet, but a pair of gloved hands had already grabbed hold of his ribs and were pulling him backwards.

Papyrus easily picked his brother up, swapping their positions and pinning sans to the sofa. His hands gripped the armrest on either side of sans' skull.

Sans blinked up at Papyrus. A trickle of fear sending shivers down his spine. But that wasn't right, was it? This was just Papyrus. Why was he nervous?

It was probably the look in his brother's eyes. He looked angry, but Papyrus was never angry. It had to be something else. Right?

"I knew if I pretended to be asleep, you would come find me. You really should have been expecting it sans. You of all people should know how good I am at setting traps."

"Ah… yeah, you sure are." Sans replied with a smile. He wanted to ask what exactly Papyrus needed a trap for, but he suspected he was about to hear a full explanation regardless.

"I suppose you're expecting me to ask you why you've been avoiding me. But the great Papyrus is very clever and I have already figured it out!"

sans shoulders hunched. All of a sudden he was feeling way too hot to be comfortable. He tugged the collar of his turtleneck down a bit, his other hand fisting the material over his ribcage.

"R-really?"

"Yes. I mean it was terribly obvious in hindsight. I'm almost mortified that I didn't see it sooner. You are, after all, my dear brother, whom I love unconditionally. I wish you would have told me, but I understand It's only natural you would try to hide it, because you were afraid of hurting my feelings. But now I'm here to tell you that you should not be ashamed! For we have a bond that could never be broken!"

Papyrus was laying on top of sans. Hipbone to hipbone. The magic in the air around them humming as two harmonious forces reunited. They were way too close. sans could hardly stand to lie still. Meanwhile, his thoughts were pin-wheeling as he tried to grasp what Papyrus was saying. He couldn't have figured it out. Please, oh god, please.

"Papyrus understands that you don't like tickle fights, and from now on I will abstain."

Oh.

Sans could almost taste his immense relief. He cracked a huge smile and shook his head.

"Yeah, sorry bro. You know me. I'm kind of sensitive."

"Indeed. That's why I always have to keep an eye out for you. I simply didn't consider that I might be the one causing discomfort."

Sans swallowed and nodded, he had no idea.

"-And Sans?"

"Yeah?"

"I'm lying through my teeth."

Papyrus' hand closed around sans' neck. His fingers easily wrapping around the top of the shorter skeleton's spinal column. Sans' train of thought didn't so much de-rail as spontaneously combust. Papyrus' smile seemed wider than should be physically possible.

"That's how you like to do it right, Pranking? It's actually kind of fun. You should have seen the look on your face!"

Papyrus let himself cackle for a moment. Meanwhile, sans was very glad he didn't strictly need to breathe, because he didn't think he could if he wanted to.

"You like playing games." Papyrus said, leaning over sans so closely that he had no choice but to meet Papyrus' eyes. "Well, I know how to play them as well, and I'm not going to let you beat me. For I am an alpha man... er skeleton!"

The soft purr that Papyrus' voice had dropped into broke at the end there. His eye ridges knitted together in confusion.

"Wait, was I supposed to say that part out loud?" he muttered. He looked at sans as if his brother might know the answer. Unable to think of anything else to do, sans simply shrugged.

"The great Papyrus is realizing he didn't think this through all the way." Papyrus murmured. He let go of Sans and sat back on the sofa. Glancing around like he was searching for something.

"Uh, give me a second..."

Sans pushed himself up on his elbows, watching as Papyrus started digging under the couch cushions. His neck still felt hot where Papyrus had been holding him and his cheekbones felt like they were on fire. A natural side effect of the bright blue blush stretching from one side of his face to the other.

Papyrus pulled a small hardcover book out from behind the back pillows and cracked it open. Post-it notes stuck out from the pages with Papyrus' all-caps scrawl all over them. The cover had an image of a man in a black suit kissing a woman in a red dress, and above them the title. 'The Alpha Male Guidebook to Complete Dominance.'

"P-Papyrus… where did you get that?" Sans asked.

Papyrus was too busy reading to hear the question. He was pouring over a paragraph, running his finger down the page as he read.

"This is the part about how to deal with unresolved sexual tension. I decoded your signals, and I waited for you to come to me. So, now we have to kiss while the moment is still heated!" Papyrus concluded, snapping the book closed and shoving it back behind the sofa.

sans gripped the armrest behind him with both hands. He really had only two defense mechanisms for dealing with stressful and unexpected situations. There was A) lay down and ignore it and B) run the fuck away.

sans tried to bolt, but Papyrus managed to grab his ankle and pull him back. He was pushed down on the sofa again, this time harder, and with Papyrus' hands tightly wrapped around his upper arms.

"No you don't! You're not going anywhere until we fix this!" Papyrus insisted. "Just hold still for a moment."

Papyrus turned his attention to sans' mouth. He started to lean in, until he realized neither of them had lips.

"You know, in hindsight, I don't think that book was written for skeletons."

sans let his skull fall back against the cushions, sighing in relief. But even as part of him felt that relief, another part was sulking in disappointment. Unfortunately, or fortunately, depending on who you asked, sans' magic had different plans.

Papyrus' desire flowed from him, circumventing sans' control to provide a solution. Something warm and hot filled sans mouth. He opened his jaws without thinking, exposing the bright blue tongue he had just gained.

Papyrus' eyes lit up.

"That's perfect! Good thinking sans!"

Papyrus took sans' lower jaw, holding his brother's mouth open. He didn't even have to think about copying the spell, it simply was. Papyrus' bright orange tongue was a bit too large to fit comfortably inside his mouth. But then again, it wasn't going to be in _his_ mouth much longer.

Teeth clicked gently against teeth and any protest sans might have offered was lost. Papyrus didn't spare a thought to easing sans into it. He simply shoved his tongue in as far as it would go.

Sans made a strangled 'Hurk' sound, his hips bucking up against Papyrus as his whole body reacted. Any deeper and the tip of Papyrus' tongue would be touching the top of his spine.

Apparently Papyrus' book hadn't provided details on what to do after this point, because he didn't move his tongue.

Sans crossed an arm over Papyrus' shoulders and pulled against him for leverage. Pressing up into the kiss and gently swiping his tongue against the underside of Papyrus'.

Papyrus squeaked. He hadn't been expecting a feeling quite that strong. He followed sans' lead, but his enthusiasm soon got the better of him. He threw more of his weight into the kiss, pinning sans down hard, and twining his oversized tongue around his brother's.

Sans muffled moan made Papyrus shiver. He thought this was absolutely wonderful. They should have been doing this ages ago.

The magic around the two monsters blead together, making it hard to tell where the lines were between them. There was the first wave of sensation, then an eco-like counterpart from the other brother. This close, it was impossible to hide. Not when their bodies, and by extension, their souls were pressed together.

Papyrus lifted his head, his cheekbones were bright orange now, complementing sans' blue. Some saliva dripped down Papyrus' chin, a side effect from the messy kisses. Both brothers panted heavily, waiting for the other to say something.

Sans swallowed, pulling the corners of his mouth back up into the mask of a smile he relied upon.

"um.. Papyrus… I don't suppose you could tell me… why you're doing this?" Sans asked.

Papyrus stared at sans. He was a very emotional monster, even at the best of times, and the passion of the kiss they had just shared hadn't dissipated one iota for him.

"You were thinking about leaving!" Papyrus blurted, his hands tightening around sans' arms. "And don't tell me you weren't, I could feel it!"

Sans blinked up at Papyrus. Guilt settling on his chest like a twenty pound weight.

"-But now you don't have to leave because we're lovers now. So we can do all the things together and you don't have to be scared anymore!"

sans' mouth hung open.

"H-how… how did you know all that?" he stammered.

Papyrus rolled his eyes in a manner that was remarkably sarcastic for him.

"I just told you, didn't I? I can feel it." Papyrus said.

Papyrus rested his chin on sans' sternum, looking up at his brother as he hugged sans' hipbone to his chest.

"I think so much clearer when I'm with you. I know I don't always understand everything that goes on the same way you do. But if I know anything at all, I know you."

Oh.

In hindsight, the revelation was so obvious sans was tempted to punch himself in the face. All this time he had been thinking about what he was taking from Papyrus, without the realizing he must be giving part of himself in turn.

The attempted prank. The eye roll. The hand around his neck. There were probably hundreds of little signals he had missed. Sans couldn't quite decide if the idea of Papyrus acting more like him was exciting or another reason to feel guilty.

"Papyrus, I'm sorry." Sans murmured.

"Good! um… what for though?" Papyrus asked.

"For a lot of things, but mostly for not being honest with you."

Sans carefully patted Papyrus on the top of his skull. He didn't quite trust himself to touch any other part. At least not until Papyrus got off his hips.

"So, I'm going to start right now, alright? Because, whatever that freaky book you got said, you don't have to do things like… uh… that, to keep me around. I'm going to find some way to deal with it. And I promise it doesn't involve leaving. Okay?"

"Really?"

Papyrus looked like he was about to go searching for the book again. sans grabbed his shoulders to stop him from getting up.

"But what if I wanted to do things with you?" Papyrus asked. "The book said there was a ton of other stuff after kissing, but I didn't skip ahead because I didn't want spoilers."

Sans' eye-lights shrunk to tiny pinpricks. The wave of emotional feedback Papyrus felt from sans might as well have spelled out the word 'yes' in ten foot high letters. But despite the protest of sans' soul, he managed to keep a straight face, and control over his mouth.

"Papyrus we can't…"

"Why?"

"We're brothers."

"So?"

Sans slapped himself in the face with both hands and let out a tortured groan. Surely the powers that be didn't hate him so much that they would make him explain incest to Papyrus.

"Fuck me." he muttered.

"That's something the book said you might say! I'm supposed to say no, for right now at least, because waiting helps build that sexual tension stuff." Papyrus replied. He took one of sans' wrists and lifted his hand away from his eye socket.

Sans let his shoulders slump and he collapsed against the sofa. He was done. Officially done with all of this.

"and where exactly did you get that book Papyrus?"

"A friend gave it to me. A friend of the little and yellow variety."

Wordlessly, sans pushed Papyrus off of him and stood up away from the sofa.

"I'll be right back. I just remembered I have something I need to do." Sans said.

Papyrus frowned and grabbed the back of sans' hoodie. Effortlessly tugging his brother back to him.

"Or we could kiss a few more times! I like that plan better."

sans' slippers lost contact with the ground, and a moment later he found himself sitting in Papyrus' lap. Gloved fingers tilted sans' chin up. His orange tongue was back, the tip running over his canines as he leered down at sans.

"Now be a good boy and do what daddy Papyrus says, alright."


End file.
